Engineering Expert Witness Blog

     Last week we looked at the arithmetic behind chemical reactions in an area of thermodynamics known as stoichiometry.  This week we’ll learn about psychrometry and the value of a summer breeze.  Well, more specifically, psychrometry involves the analysis of gas and vapor mixtures, like air and water. 

     You may not have ever heard of psychrometry or psychrometrics, but your body is familiar with it.  In fact, it adheres to its principles every time it sweats.  See, sweating keeps you cool, and that’s because when liquids like water evaporate, they absorb heat in the process.  When sweat, which is mostly water, evaporates from your skin, it takes some of your body heat with it, dissipating it into the atmosphere.  That’s why a roomful of sweaty bodies is so uncomfortable to be in.

     Now let’s say you’re outside and it’s a hot, humid summer day.  The air already contains a lot of moisture, so it can’t absorb as much sweat from your body as it would in a drier environment.  As a result, your sweat doesn’t evaporate so well.  It lingers on your skin, keeping you miserable.  Now introduce a summer breeze.  The increase of air flow across your skin that it produces serves the same purpose as an electric fan in your home.  They both make you cooler by increasing the surrounding air flow, thereby making more air available to contact your skin, and increasing the sweat evaporation process.

     In the study of psychrometry, mechanical engineers learn about the thermodynamic properties of moist air.  Then they use these properties to analyze conditions and design processes which deal with moist air, things like air conditioning systems and dehumidifiers.

     Let’s return for a moment to that air conditioner example that we used in our discussion of Thermodynamics in Mechanical Engineering, Part III. This is shown in Figure 1 below.  Psychrometry would be used here, too.  For example, when you are determining how much heat must be removed from the warm, humid air inside your home by the evaporator coil inside your air conditioner.  Knowing how much heat must be removed is one of the first steps to designing a system which is properly sized and works efficiently in order to keep you comfortable.

Figure 1 – A Simple Refrigeration Cycle

     Psychrometric calculations can get pretty involved, and our discussion is meant to provide only a brief overview, but suffice it to say that their basic function is to set up a mass and energy accounting system that adheres to the principles of the First Law of Thermodynamics.  In other words, energy and mass going into a system has to add up to energy and mass coming out.

     Now, let’s return to our discussion on psychrometry in relation to the design of the air conditioning system of Figure 1.  Let’s focus on the evaporator coil from this system, as shown in Figure 2.  This coil is contained in a duct along with a blower.  The air sucked into the evaporator coil from the room has water vapor mixed into it.  The pure air part and the water vapor part each contain heat energy.  Our bodies perceive that heat energy as warm, humid air.  As that humid air is cooled by the evaporator coil, much of the water vapor condenses out of it as liquid moisture, which is then drained out of the air conditioner.  What’s left is a cooler mixture of air and greatly reduced water vapor.  This mixture then leaves the evaporator coil and is sent back into your home from the duct by way of a blower, resulting in a more comfortable environment for you.

Figure 2 – An Evaporator Coil In An Air Conditioning Unit 

     So, using the First Law of Thermodynamics, the heat accounting system for the air conditioner looks like this:

 Q_evaporator =

      (Q_air + Q_{water vapor})_{going in} – (Q_air + Q_{water vapor} + Q_{condensed moisture})_{going out}

where, “Q_evaporator” is the heat energy removed by the evaporator coil, “Q_air” is the heat energy contained in the air, “Q_{water vapor}” is the heat energy contained in the water vapor, and “Q_{condensed moisture}” is the heat energy contained in the condensed moisture drained out of the air conditioner.   By the way, the letter “Q” is often used to denote heat in thermodynamics.

     To solve for the equation above, one has to first consider what the pressure, temperature, and relative humidity of the air will be in the room when the air conditioner is first turned on.  We must next determine what the desired pressure, temperature and relative humidity should ideally be once the conditioned air leaves the evaporator coil on its journey back into the room.  In other words, we need to know the conditions we are starting out with in order to know where we want to end up, comfort-wise.  Once these parameters are known, thermodynamic formulas are used to calculate how much heat must be removed by the evaporator coil.  Now the air conditioning equipment can be designed with a large enough evaporator coil, with sufficient refrigerant flowing through it, and a large enough blower to efficiently perform the task of keeping us cool.

     This concludes our tour of the world of thermodynamics.  Next week we’ll begin our discussion of an area of mechanical engineering known as fluid mechanics, which is the study of the force, pressure, and energy of both stationary and moving fluids.  We’ll see how a hydraulic car jack works, how water flows through pipes, and how airplane wings lift a plane into the sky.

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